1. Field of the Invention
The present invention relates to a charging member used for an electrophotographic apparatus, etc., and particularly to a charging member for charging, transfer, and the like to which a voltage is applied to charge the surface of a member to be charged, and an image forming apparatus using the charging member, such as an electrophotographic apparatus, or the like.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographlc apparatus (a copying machine, an optical printer, or the like), an electrostatic recording apparatus, or the like, a corona charger is used as a device for charging the surface of an image supporting member as a member to be charged, such as a photosensitive member, a dielectric member, or the like.
The corona discharger is effective as means for uniformly charging the surface of the member to be charged, such as an image supporting member or the like, to a predetermined potential. However, the corona discharger requires a high-voltage power source for corona discharge, and thus produces undesirable ozone.
In contrast to the corona discharger, a contact charger comprising a charging member to which a voltage is applied is brought near or in contact with the member to be charged to charge the surface of the member to be charged, permitting an attempt to decrease the voltage, and causing the advantage of generating less ozone.
Such a charging member requires low conductivity for obtaining uniformity in the member to be charged, such as a photosensitive member or the like, and for preventing leaks due to pinholes, flaws, etc. in the surface of the member to be charged.
For example, a transfer roller used in an image forming apparatus such as a copying machine or the like is a member for transferring, to transfer paper, a toner image on the image supporting member such as the photosensitive member, an intermediate transfer member, a transfer drum, or the like. When the transfer roller is brought into pressure contact with the photosensitive member for developing the toner image through the transfer paper as a material to be charged, a charge with polarity opposite to the toner of the toner image is supplied to transfer the toner image on the photosensitive member to the transfer paper by adsorption. However, the density of the charge supplied to the transfer paper significantly affects image quality.
Namely, with a low charge density, toner adsorptivity is weak, and particularly, black spots around a line image occur on dry paper. With a high charge density, bleeding occurs due to reverse polarity charging of the toner, and a high quality image cannot be obtained.
A nonuniform charge density, density nonuniformity occurs during transfer of a solid black image, or spot-shaped transfer nonuniformity such as a sand-like ground or the like occurs. Therefore, the surface of the transfer roller preferably has uniform conductivity.
In order to satisfy this requirement, a rubber roller is used, in which conductive particles of carbon black, graphite, a metal oxide such as titanium oxide, silver oxide, or the like, a metal such as Cu, Ag, or the like, or particles, which are made conductive by coating the surfaces of these particles, are mixed and dispersed on a conductive core material made of a metal or the like.
However, the resistance of the rubber roller comprising the dispersed conductive particles is difficult to control the results in variations in local resistance due to nonuniform dispersion of the conductive particles, causing charging nonuniformity or breakage of the photosensitive member due to partial leaks.
On the other hand, a method of obtaining conductivity is proposed, which uses NBR (acrylonitrile butadiene rubber) or hydrin rubber. Particularly, this method is capable of obtaining good uniform conductivity without a conductive additive. However, such a polar polymer contains a crystalline portion in its molecule, and thus produces significant permanent deformation due to its fluidity, causing deformation of the contact portion of the roller after standing for a long period of time. This causes an image defect due to an excessive current from the nip portion, and nonuniformity in carrying a transfer material such as paper or the like to cause nonuniformity in an image.
Namely, NBR and hydrin rubber contain cyano groups containing chlorine with high polarity. Therefore, polymer chains are attracted to each other through such polar groups to form the crystalline portion having a structure in which the polymer chains are regularly arranged. In the crystalline portion, the polymer chains are not cross-linked. In the amorphous portion, the polymer chains are cross-linked, and even if it is deformed by external force, it returns to the initial shape by releasing the external force. However, in the crystalline portion, the polymer chains are not cross-linked, and thus cannot be return to the initial shape, readily causing permanent deformation. Therefore, when a charging roller made of rubber having such polar groups is used as a transfer charging roller, and when the charging roller is brought into contact with a photosensitive drum which is not rotated for a long time, the contact portion of the charging roller is deformed, and the width of the nip between the charging roller and the photosensitive drum is increased, causing an excessive current to flow between the charging roller and transfer paper. As a result, the surface of the photosensitive drum is charged, and toner adheres to the charged surface during development to cause image nonuniformity.
In the deformed portion of the charging roller, the conveyance speed of the transfer paper is changed, causing nonuniformity in an image.
A polymer having high polarity, such as NBR or the like, has high adhesion, and thus adheres or sticks to the member to be charged, such as the photosensitive member or the like, when allowed to stand in contact with the member to be charged for a long period of time. This causes the problems of transferring the components of the charging member onto the member to be charged, and stopping rotation of the roller due to adhesion of the polymer.